This invention relates to circuitry for regenerating synchronizing signals obtained from a composite television video signal.
In television systems, the synchronizing signal components of the composite video signal often become distorted and subject to noise during signal processing which results in erratic synchronism of the television picture. It is common practice, in such cases, to separate the synchronizing signals from the composite video signal and to replace the original synchronizing signals with a regenerated synchronizing signal to improve the overall picture synchronization.
Known circuits for separating and regenerating the synchronizing components from a composite video signal generally comprise a level detector and clamp circuit followed by a monostable multivibrator. In operation, such circuits generally detect the leading edge of the synchronizing pulse and, in turn, trigger the monostable multivibrator, which generates a replacement pulse of appropriate duration and amplitude. These known circuits, however, often do not produce reliable and uniform signal regeneration because the leading edge of the original pulse may itself be distorted or noisy, which will offset the detection, timing and initiation of the monostable multivibrator. Additional difficulties in reliable and uniform signal regeneration occur in the prior art circuitry in situations where the signal amplitude level applied to the sync separator varies substantially in amplitude as is common in switching between several video signal sources. Substantial changes in signal amplitude are particularly troublesome in developing accurate timing regneration due to the nature of the synchronizing signals, in particular, the horizontal synchronizing signal which has a well defined rise time in accordance with the NTSC standards. Substantial changes in video signal level will result in a substantial change in average picture level and a resulting shift in the time at which the threshold level of the sync separator is activated. Therefore, even though the output amplitude of the sync regenerator circuit is amplitude controlled, the timing error in initiating sync signal regeneration is coupled through to the newly regenerated signal. This timing error is particularly troublesome when one or more video signals are coupled together in a manner known as genlock, wherein the synchronizing components of a master signal are substituted for the synchronizing signals of a slaved video signal. Such timing errors are also troublesome in situations where one or more video signals are intermixed for special effects.